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std::empty

Defined in header <array>
Defined in header <deque>
Defined in header <forward_list>
Defined in header <iterator>
Defined in header <list>
Defined in header <map>
Defined in header <regex>
Defined in header <set>
Defined in header <span> (since C++20)
Defined in header <string>
Defined in header <string_view>
Defined in header <unordered_map>
Defined in header <unordered_set>
Defined in header <vector>
(1) 
template< class C >
constexpr auto empty( const C& c ) -> decltype(c.empty());
 (since C++17)
(until C++20)		 
template< class C >
[[nodiscard]] constexpr auto empty( const C& c ) -> decltype(c.empty());
 (since C++20)
(2) 
template< class T, std::size_t N >
constexpr bool empty( const T (&array)[N] ) noexcept;
 (since C++17)
(until C++20)		 
template< class T, std::size_t N >
[[nodiscard]] constexpr bool empty( const T (&array)[N] ) noexcept;
 (since C++20)
(3) 
template< class E >
constexpr bool empty( std::initializer_list<E> il ) noexcept;
 (since C++17)
(until C++20)		 
template< class E >
[[nodiscard]] constexpr bool empty( std::initializer_list<E> il ) noexcept;
 (since C++20)

Returns whether the given range is empty.

1) Returns c.empty().
2) Returns false.
3) Returns il.size() == 0.

Parameters

c - a container or view with an empty member function
array - an array of arbitrary type
il - an initializer list

Return value

true if the range doesn't have any element.

Exceptions

1) May throw implementation-defined exceptions.

Notes

The overload for std::initializer_list is necessary because it does not have a member function empty.

Feature-test macro Value Std Feature
__cpp_lib_nonmember_container_access 201411L (C++17) std::size(), std::data(), and std::empty()

Possible implementation

First version 
template<class C>
[[nodiscard]] constexpr auto empty(const C& c) -> decltype(c.empty())
{
    return c.empty();
}
Second version 
template<class T, std::size_t N>
[[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept
{
    return false;
}
Third version 
template<class E>
[[nodiscard]] constexpr bool empty(std::initializer_list<E> il) noexcept
{
    return il.size() == 0;
}

Example

#include <iostream>
#include <vector>
 
template<class T>
void print(const T& container)
{
    if (std::empty(container))
        std::cout << "Empty\n";
    else
    {
        std::cout << "Elements:";
        for (const auto& element : container)
            std::cout << ' ' << element;
        std::cout << '\n';
    }
}
 
int main()
{
    std::vector<int> c = {1, 2, 3};
    print(c);
    c.clear();
    print(c);
 
    int array[] = {4, 5, 6};
    print(array);
 
    auto il = {7, 8, 9};
    print(il);
}

Output:

Elements: 1 2 3
Empty
Elements: 4 5 6
Elements: 7 8 9

See also

(C++20)
checks whether a range is empty
(customization point object)

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